How do i add a certain amount of objects without lowering the frame rate?
There are many factors that determine frame rate.

Computer...Particles...

The complexity of the scene is one such and probably the reason why you are losing frames.

You can check the simplify box, but I for one wouldn't rely on that, it may help but not by much.

You can view everything in wireframe.

Another method is to use proxy characters in place of the 'final' characters.

Another method, that I use is to not use the in-blender time to base your animation judgment, instead take a video of yourself doing said action and place it in the bg, you can use that as your basis of time, action and reaction if you so require. 

A good addon to said methods would to be to render the animation at a low resolution opengl render (after you had done...) that way you can check that the animation is too your liking without waiting for the long render times.